Supercapacitors represent very promising components in the field of electrical energy storage. Many applications are currently being developed as peak-power sources in high-power applications such as hybrid vehicle motors or fuel cell powered vehicles. In applications like these, power supplies are needed and also regenerative braking requires to charge storing devices under high currents.
A supercapacitor module generally comprises a plurality of individual supercapacitors which generally all have the same nominal electrical parameters (same capacity, same internal resistance) and are all connected in series. This is because the nominal voltage of an individual supercapacitor is low, typically in the range of 2.5 Volt. Because the applications mentioned above generally require voltages exceeding a few tens of volts, or even a few hundreds of volts, a number of individual supercapacitors are connected in series to provide a module fulfilling the specifications of use for instance in automotive applications.
In charging mode for instance during regenerative braking as well as in discharging mode during power supply for moving the vehicle or other use of electrical energy, the current is by definition identical in all the individual supercapacitors, because the supercapacitors are connected in series. The voltage at the terminal of each individual supercapacitor should also be identical. However, it is known that there is a spread of the characteristics—capacitance for example—of the supercapacitors relative to each other, due to manufacturing tolerances and/or due to different aging of the individual supercapacitors, and possibly to a temperature gradient within the module, due to its environment. This leads to different ends of charging voltages for each of the supercapacitors. Also, different leakage currents for each of the supercapacitors of the module can cause the voltages of the single cells to diverge.
This problem compromises correct operation of the supercapacitor module. Some supercapacitors of the module may reach voltages exceeding their nominal charging voltage, which degrades their characteristics and leads to premature aging or even to failure. Thus, because the supercapacitors are connected in series, the module as a whole cannot function correctly.
To solve this problem, it is known to design bypass circuits connected in parallel with the terminals of each individual supercapacitor of a module comprising a plurality of supercapacitors. The general principle of known supercapacitor balancing systems consists in bypassing some or all of the supercapacitor charging current in order to equate the final charging voltage to a predetermined identical value on the terminals of all the individual supercapacitors.
The bypass circuits can be based on resistors each connected in parallel to the individual supercapacitors, and all connected in series. This solution is the simplest one, but it is energetically inefficient, because current in the resistors generates many losses.
It is also known to connect Zener diodes across each supercapacitor, to limit the maximum voltage value of the supercapacitor, for example at 2.5 Volt. This solution is energetically efficient as far as all voltages are less than the limit value, but the power dissipation can become important if many supercapacitors reach their maximum limit voltage.
Other solutions are based on more sophisticated methods, in order to minimize the energy consumed during balancing operation. It is also known to design active circuits for voltage balancing, as illustrated by U.S. patent application 2003/0214267. But these methods require additional electronic components, which mean additional costs and weight. In case of electronically active or passive balancing devices, the approach described above results in enhanced self discharge of the module.